Intel IXP1200 manual CRC-32 Hardware Generation on Transmit, Transmit Alignment

IXP1200 Network Processor Family ATM OC-3/12/Ethernet IP Router Example Design

•Upon reception of the first cell, data11 is saved in the VC cache/table entry. Upon reception of the 2nd cell, data11 is retrieved from the VC cache/table entry, combined with data0 of the second cell, and written in a single burst to DRAM.

Moving the nth cell (not cell0) in a PDU from the RFIFO to DRAM is similar to using the macro atm_move_cell0_rfifo_to_sdram(), except that:

•The nth cell must start with a run-time crc_residue resulting from CRC on the previous cell in the PDU.

•The nth cell must combine data11 of the previous cell with data0, as shown in Figure 17.

3.6.2CRC-32 Hardware Generation on Transmit

Figure 18 and Figure 19 show the layout of the cell source as it appears in DRAM, and the desired format in the TIFO, respectively. Aspects of the first, nth, and last cell are all overlaid on the same diagram, as the positions are the same. In each diagram, rows are 64-bit “quadwords”.

Figure 18. Transmit cell as seen in DRAM

	0	1	2	3	4	5	6	7 Bytes -> (Big Endian Diagram)
0	-	-	-	-	-	-	LLC
1	LLC						IP
2	IP
3	IP
4	IP
5							AAL5
6	AAL5		CRC32*				CellN+1

Figure 19. Transmit cell seen in TFIFO

	0 1 2	3 4		5	6	7 Bytes -> (Big Endian Diagram)
0	ATM Header		LLC
1	LLC1		IP
2	IP
3	IP
4
5			AAL5
6	CRC32		-	-	-	-

3.6.2.1Transmit Alignment

The alignment of this cell in DRAM is dependent on how the data was received. In this example design, the data was received on Ethernet, with a 14 byte Ethernet header. Therefore, the first byte of the IP header starts on the 15th byte of the buffer.

The sdram_crc[t_fifo_wr] commands account for this alignment by using the IXP12xx byte alignment hardware. These diagrams show bytes in big-endian order, while the instruction encoding asks for byte alignment assuming little endian order. Therefore the 6-byte offset shown here, becomes a 2-byte offset as encoded in the indirect_ref.

Application Note 27

Modified on: 3/20/02,